Portable electronic devices are conventionally powered by batteries. With the advancing sophistication of such devices, and an ever increasing need for more power, improvements have been made to the shelf life, efficiency and overall useful life of batteries in recent years. Notwithstanding these improvements, batteries are fundamentally limited in terms of power generation per unit volume, energy storage per unit mass and disposal of the ultimate by-products of power generation, e.g. toxic metals.
The use of combustion processes for the generation of electrical energy has been recognized in the prior art as having a number of advantages over batteries. Hydrocarbon fuels provide an energy storage density of between 40 and 50 MJ/kg, while lithium ion batteries, for example, have an energy storage density of 0.4 MJ/kg. Even at comparatively low rates of efficiency in converting thermal energy resulting from the combustion of hydrocarbon fuels to electrical energy, the energy storage density is much higher using hydrocarbon fuels in comparison to batteries. Further, hydrocarbon fuels are readily available, easily stored and have a longer shelf life than batteries. Additionally, the by-products of combustion are primarily carbon dioxide and water which do not present disposal difficulties.
U.S. Pat. No. 6,613,972 discloses a micro-generator system designed to generate electrical energy on a scale sufficient to power portable electrical devices and micro-electro-mechanical-systems (“MEMS”) using a micro-combustor which operates with hydrocarbon fuel. Propane, butane or methylacetylene combined with air is supplied to the micro-combustor where it is burned within a combustion region to produce heat. One or more internal walls of the micro-combustor are formed of a thermoelectric material which is capable of producing electrical energy when exposed to a temperature differential. This temperature differential is created by directing a flow of incoming, cooler air-fuel mixture, and a flow of the heated exhaust gas produced from combustion, past opposite surfaces of the thermoelectric material.
The micro-generator of the '972 patent has a number of the advantages described above pertaining to the use of hydrocarbon fuels to generate electrical power. Nevertheless, its efficiency in converting thermal energy to electrical energy is on the order of about 5%, and no provision is made to convert exhaust gases from combustion in the micro-combustor to electrical energy. This limits the amount of power which can be obtained from the device, and therefore restricts the types of portable electronic devices and MEMS with which it can be used. Additionally, fuel consumption can become an issue given such a low conversion efficiency. Consequently, the micro-generator of the '972 patent has limited practical application in its present form.